1. Field of the Invention
The present invention relates to integrated magnetic transducers. It is particularly applicable to the reading and/or writing of data contained on magnetic recording carriers such as rigid or flexible discs and tapes.
2. Description of the Prior Art
It is known that, in order to record or write data on a magnetic recording carrier, at least one modification of one of its magnetic properties is created on (or in) the carrier at a plurality of precisely determined locations. This modification is translated by at least one variation of the physical magnitude which characterises the property. The reading of this data is effected by detecting the modifications and by transforming the variation of the physical magnitude into a variation of another physical magnitude which is most often the variation in the voltage or current of an electric signal.
The magnetic property used to record data on a magnetic carrier is defined, for example, either by the absence or the presence of a magnetic induction or by the sign of the latter or again by its direction.
It is known that magnetic discs carry data on concentric circular recording tracks which have a radial width not exceeding a few hundredths of a millimeter and generally covering the greater part of their two faces. Magnetic tapes on the other hand carry data on tracks parallel to the length of the tape.
The most frequently used means, which enable either the recording of data on carriers such as discs or tapes or the reading of data, or possibly to carry out both of these two functions, are called "magnetic transducers." Generally, one or more transducers are associated with a recording carrier, the carrier moving in front of it or them. For simplification it will be assumed in the rest of the text that a single transducer is associated with the same carrier.
A magnetic transducer comprises a magnetic circuit around which is disposed a winding and which comprises an air-gap. The air-gap is disposed at a very short distance from the surface of the carrier of the order of a few tenths of microns. The winding comprises two electric input and/or output wires.
In order to record data on the carrier associated with the transducer, the winding is supplied by an electric current of which the direction or duration of passage can be varied in the winding. The carrier is thus submitted to the magnetic field of intensity and variable direction created by the transducer in the immediate neighborhood of its air-gap (at a few tenths of microns from the latter), which creates a succession of small magnetic areas on each track of the carrier, the size of which areas is of the order of that of the air-gap and which has magnetic inductions of opposite directions. These areas, also called "elementary magnetic areas," are distributed over the entire length of the track.
Conversely, when the data of a given carrier moves past the air-gap of the transducer which is associated with it, the transducer delivers electric signals by means of its electric input and/or output wires, which signals are transmitted to the electronic read circuits associated with the transducers.
The current tendency in the development of magnetic transducers is to produce, according to integrated circuit manufacturing techniques, transducers which are smaller and smaller in size having air-gaps whose dimensions are of the order of the microns. Such transducers are known in the art and are manufactured, for example, by Compagnie Internationale pour l'Informatique CII-Honeywell Bull. Reference may be made to U.S. Pat. Nos. 3,723,665 and 3,846,262 for a description of such magnetic transducers and their method of manufacture.
Other patents related to thin film magnetic transducers to which reference may be made for a better understanding of the invention include U.S. Pat. Nos. 3,344,237; 3,634,933; 3,848,212; 3,879,760; 4,052,749; 4,012,282; 4,072,993; 4,092,688 and RE 29,326. Reference may also be made to IBM Technical Disclosure Bulletin Vol. 14, No. 7, Dec. 1971 of Kehr and Thornley which describes and illustrates a process for fabricating a magnetic recording head.
A transducer of this type comprises, on the one hand:
(a) a magnetic circuit formed by two thin magnetic layers connected at one end in such a manner that they are magnetically coupled, and disposed at the other end adjacent the recording carrier associated with the transducer, so as to form an air-gap. The air-gap is usually situated at a few tenths of microns from the carrier. The air-gap, which is more or less of rectangular shape, is very much longer than it is wide, and approximates the width of the carrier tracks. The carrier is moved more or less perpendicular to the layers which constitute the pole pieces of the transducer. One of the two thin magnetic layers is disposed on a substrate of insulating material and on the other hand: PA1 (b) an electric coil formed between the thin magnetic layers, by thin conductive layers which are superimposed in a direction perpendicular to the plane of the thin magnetic layers. The conductive layers form an envelope containing the conductive layers which are separated from each other by thin electrically insulating layers.
It should be noted that the term "thin layers" designates layers whose thickness is of the order of a few Angstroms to a few microns.
The magnetic recording carrier associated with the transducer moves past the latter perpendicular to the plane of the two magnetic layers, that is to say perpendicularly to the length of the air-gap. During this movement, any magnetic area of a carrier track opposite which the transducer is located, passes successively at right angles with the first magnetic layer which is called an "upstream pole piece" and with the second magnetic layer which is called a "downstream pole piece." Stated another way, the magnetic area successively encounters the upstream and downstream pole pieces or the carrier successively encounters the upstream and downstream pole pieces.
Of the two pole pieces, it is the downstream pole piece which defines the nature of the data recorded on the carrier, that is to say principally the direction and the module of the magnetic induction in each of the magnetic areas created on this carrier. In effect, when the carrier moves past the transducer, each of the areas is submitted successively in time to the magnetic field created by the upstream pole pieces in the immediate neighborhood of its surface, on the side of the transducer air-gap and to the field created by the downstream pole piece, which therefore acts last on the said area.
In other words it can be said that, when data is recorded on the carrier, only the downstream pole piece leaves its "field imprint" on the carrier. Preferably the downstream pole piece of an integrated transducer is that which is placed on the substrate.
In current practice, the coil of an integrated transducer comprises two windings which are identical, that is to say each winding has the same number of turns and has a common point at the center top. The first winding, designated the BC winding (with the reference to FIG. 1) is the winding whose thin conductive layers are closest to the downstream pole piece. The second winding designated the BA winding is the winding whose thin conductive layers are furthest away from the downstream pole pieces. The point B identifies the electric point common to the two windings, and points A and B identify the remaining ends or terminals. The coefficient of mutual magnetic coupling of the first winding BC with the downstream pole piece, also called the coefficient of mutual inductance, is greater than the coefficient of mutual magnetic coupling of the second winding BA with the latter.
This difference in mutual coupling produces the following consequences. It is known that in order to record two successive elementary areas on a track of the recording carrier, one supplies successively the first winding BC with a voltage and/or current pulse, for example a positive one, (the second winding BA is then not fed), and the second winding with a voltage and/or current pulse for example negative (the first winding BC not then being fed). The amplitudes of the negative and positive pulses being equal in absolute value. The coefficients of mutual magnetic coupling of the windings BC, BA with the downstream pole piece being different, the result is, principally, that the modules of the magnetic inductions created by the transducer within the two successive elementary areas recorded are different. The module of magnetic induction in the elementary area recorded by the passage of the electric current in the winding BC is greater than the module of magnetic induction in the elementary area recorded during the passage of the electric current in the winding BA, since the magnetic coupling of the winding BC with the downstream pole piece is greater than that of the winding BC with this same piece.
In other words, a curve representative of the magnetic induction measured along a track of the carrier is asymmetrical with respect to the ordinate or Y axis M=0 where M is the magnetic induction. The magnetic induction occurs as a succession of magnetic induction pulses which are alternatively negative and positive, the module of the identical negative pulses being different from the module of the identical positive pulses.
It could again be said that the magnetic induction in the recording carrier is asymmetrical.
When two successive elementary magnetic areas of a track of the carrier go past the transducer, the transducer supplies an electric pulse such that the time interval separating the instant (t.sub.1) where its voltage is zero from the instant (t.sub.2) where its voltage is maximum is substantially different from the time interval separating this same instant (t.sub.2) from the instant (t.sub.3) where its voltage returns to zero. It is clear that the same applies for each electric pulse supplied by the transducer when all the elementary areas of a track travel past it. It can then be said that the signal for reading data on a track is asymmetrical as a function of time.
This asymmetry of the signal has disadvantages with respect to exploitation of the signal by the electronic circuits associated with the transducer and can cause errors in determination of the value of data which each of these pulses represents. This risk of error is greater as the density of data along each track of the carrier becomes greater, that is to say when the number of elementary magnetic areas per unit of length measured along the circumference of the track is greatest. In actual current practice, high density data recording carriers are more and more frequently used and thus the problems relating to risk of error have become more important with advancements in transducer technology.